Lemna spp., Spirodela spp. and Wolffia spp. (referred to as duckweed) are considered native and ubiquitous plants in South Africa that are becoming a growing concern in slow moving water bodies due to their extremely rapid growth. Duckweed populations often dominate water bodies, in some cases with 100% cover, resulting in trickle down effects in the ecosystem. These are very similar problems to those commonly associated with other invasive plants such as water hyacinth, Kariba weed and water lettuce. An understanding of the drivers of duckweed population growth and associated increases in biomass is a key to the development of management strategies for these systems. Several physicochemical as well as biological aspects of the Bushmans (33˚18’45 S, 26˚ 04’

45 E) and the Bloukrans (33˚19’25 S, 26˚ 36’ 00 E) river systems in the Eastern Cape were studied. These included macroinvertebrate diversity, abundance and functional feeding groups as well as basic water parameters and nutrient analysis to gain an understanding of whether the populations are bottom-up (nutrients) or top-down (herbivory) driven. Preliminary results indicating a lack of macroinvertebrate herbivores and herbivory in conjunction with relatively high nutrient levels suggests that populations of duckweed in these two river systems are likely to be driven by bottom -up mechanisms rather than top-down herbivory. Management of this nuisance plant may then require a reduction in available nutrients in problematic areas.